We bought our Mac in November 2009. The only people that uses it is me and my 12 year old daughter. Almost daily my daughter changes the wallpaper with internet pictures of her favorite bands. I'll tease her by changing the wallpaper picture to a goofy picture from the internet. Two days ago I changed her band picture with a picture of a cartoon cat. When she saw the pic, she changed it back to the band picture. Yesterday, when she was at school, I went on the Mac. Her band picture was still there. After I was done, I turned off the internet, the cartoon cat was the wallpaper. Later, I went back on the computer, the band picture was back. The band pic was still there when I was done. When my daughter got home from school she went on the Mac. The wallpaper was now one of the stock photos that came with the Mac. My daughter changed it back to her band picture. It's been fine ever since. Also, my daughter has a lot of videos she made with her camera and put them on the Mac. I had about ten videos I downloaded off Youtube. Yesterday, my daughter's videos were there, but my Youtube videos were gone. Any thoughts? Note...we never turn off the Mac. We just let it go to sleep on it's own.